I don't know. And why did it take until book 25 before Erek and the Chee started covering for them at home?
And why did it take until book 51 until they decided to morph ducks for endurance flight? It would've been very practical for many missions, and it would allow them to fly to places in formation, which is convenient.
Why didn't Tobias get an owl morph? It would allow him to help significantly more during night missions, spy for more hours, and as living as a bird, he wouldn't have an issue using the body.
Why didn't they hoard and use Dracon beams? It would be far more effective in combat in general.
There is an endless list of "Should Have's" in this series. Things that would have made life easier for them, or increased their effectiveness. Still, we can't dwell on them.
Back on topic, I don't really agree. If it got out that they were lying, it would earn the resent of the new Animorphs, and resent between soldiers is not a good thing. I feel that adding people to the ranks would have been necessary, but it would be a very touchy thing to do. Personally, I would try not to add people in groups, and try to pick people without very large families or very large groups of friends. That would be for two reasons - if they join alone, it promotes bonding between the group as a whole instead of the formation of cliques, and smaller families are statistically less likely to have controllers in them. As we can see from Marco and Jake, having a controller of a person you love can seriously warp one's motives.
I would not just add people based on stereotypes or expectations (The handicapped kids...), but rather after intense research, and unfortunately, spying. Know they are not a controller, know they they do not have multiple responsibilities that could raise suspicion if interrupted, and even the right personality type. Make sure their families are not full of controllers. Follow people.
As you can see, this would be a large task that would take many hours and shifts, and leads to another "should have". I would have 'disappeared' far earlier. Set it up. Use morphs and the Chee's holograms to make deaths or other excuses seem realistic, and separate the events between the Animorphs by at least a month, as to not raise suspicion. This includes immediate family, and as such, kidnapping the controllers.